Here are some tips and observations I picked up from more experienced hitchhikers, on a trip from Atlanta to San Francisco, and from many trips within New York and New England
The first aim of hitchhiking is to reach your destination in one piece, so some guidelines for safety should be established, and followed religiously (you are of course free to make your own, but follow them just the same).
Most importantly, always be ready to turn down a ride. This may be hard when you've been standing in the same place for 7 hours, but it's absolutely essential. If you feel uncomfortable for any reason, just say "no thanks."
If your driver asks whether you have money for gas, food etc., say no. This may lead to awkward moments when they insist on buying you a meal, but you should never reveal that you're carrying any cash at all. If someone pulls over and asks about money right off the bat, you should probably pass on the ride.
If at all possible, keep your bag(s) with you in the car. If you allow them to go into the trunk you may either lose them, or find it difficult to extract yourself from a sketchy situation.
Consider bringing a friend. This will slow you down, but you'll still get there eventually, and will be much safer. I would not recommend that women hitch alone under any circumstances.
I would not advise carrying a weapon, you will probably be picked up by the police at some point, if you don't give them a reason to arrest you they'll probably just drive you well out of your way and leave you alone.
A shotgun in a pickup truck's gun rack is not necessarily a bad thing, a pistol on the front seat is.
If at all possible, avoid California. The preponderance of psychos makes hitchhiking both dangerous and slow.
Some more general tips and observations:
The sign is essential. The advertisements in the front of newspaper vending boxes are made out of a nice plastic material. You can make your letters from strips of duct tape and have a sign that you can reuse for days. Depending on where you are in your trip, your sign may have the name of the next highway you're trying to reach, a city or state, or simply "west" or "east".
Don't forget your can opener, the Boy Scout type is small and unlikely to be construed as a weapon. You can live extremely cheaply on canned beans and peanut butter.
Short hair and a US Army field jacket will get you a lot of rides from former servicemen who "did a lot of hitchhiking when they were in the army."
Expect 80% or more of your rides to be in pickup trucks. I have no idea why, but pickup drivers seem to be extremely nice.
Be prepared to do a lot of talking. These people are doing you a favor, and taking a risk, so be nice and play along. You will definitely meet a lot of Christians, if you're an athiest you'll be doing them a big favor by letting them think they're getting through to you. This is also not a good time to bring up any provocative political ideas, e.g., your thoughts on terrorism, John Brown, and Crazy Horse.
Eventually you will be picked up by a 40-something woman in a minivan. Take pains to make her comfortable, don't bother to point out that she's an idiot for picking you up.
You are likely to have some strange experiences in truck stop bathrooms. These will make for good anecdotes someday, and if you're so inclined, "you could make a lot of money through that hole."
If your mom does know what you're doing, make sure you check in with her regularly, she's probably pretty worried.
You will meet a lot of very friendly people, and some real characters, so have fun and be safe.